Thermionic cathode



y 1937- P. T. WEEKS THERMIONIC CATHODE Filed June 1, 1931' Fig 2" Paul TWee/fis INVENTOR ATTORNEY Patented May 4, 1937 1 UNITED STATES rarsur QFFICE THERMIONIC CATHODE- Paul T. Weeks, Cambridge, Mass, assignor to Raytheon Production Corporation, Newton, Mass., acorporation of Delaware Application June 1, 1931, Serial No. 541,155

8 Claims. (01. 250-475) This invention relatesto thermionic cathodes vided with two holes I and B through which pass of the indirectly heated type. Among the obthe two ends land of the filament I. This jectsof this invention is the provision of a catharrangement firmly secures one end of the filaode in which disturbing effectswhich ordinarily ment in place. The cylinder l3 carries an inoccur during the use of such a cathode aresubsulating bushing I! in the top thereof. This =5 stantially reduced. Another object is to produce bushing is provided with a longitudinal slot H such a cathode that is adapted to be handled into which is inserted a fiat loop 3 formed at the easily during the manufacture of a tube conupper end of the filament l. The slot I2 is of taining the cathode, and which may be supsubstantially the same width and thickness as w ported in that tube in a simple and efiicient the loop 3, but preferably extends throughout manner. the length of the bushing H. In this manner The foregoing and other objects of this invenany transverse motion of the upper end of the tion will be best understood from the following filament is prevented. I have found that it is description of an eXemplification thereof, refdesirable to arrange for some longitudinal moerence being had to the accompanying drawing, tion of the upper end of the filament, due to the 5 wherein? j; fact that intermittent heating" and cooling of Fig. 1 is a side View of the completed cathode; the filamentduring its ordinary use causes an Fig.2 is a cross-section of the cathode shown appreciable elongation thereof. It, will be seen in Fig. 1 taken along a plane at right angles to that the slot and loop arrangement of the bushthe plane of the view as shown in Fig. 1; ing H and the filament l allows the upper end 20 Fig. 3 is an exploded perspective view of the of the filament to move longitudinally, thereby elements of the cathode; and taking care of this elongation.

Fig. 4 is a view showing the cathode supported The filament I is formed preferably of some. in assembled relation with the other electrodes refractory material, such as tungsten or molybm; of the vacuum tube. denum. Upon heating current passing through My. cathode consists primarily of a thin metal the filament, its temperature rises to such a cylinder l3 within which is supported a double point that the material of which it is made behelix filament I. The cylinder 13 is made of comes relatively soft andflexible. Unless some thin metal, preferably nickel or iron, and is promeans were taken to prevent it, distortion and 20 vided with a coating II, which when raised to sagging of the filament itself would occur. Such the proper temperature becomes a good emitter distortion would ordinarily be sufficient for the of electrons. In cathodes of this type, it is desirfilament to come into contact with the walls of able that upon supplying a heating current to the cylinder 13. I avoid this particular difthe'filament I, the temperature of the cylinder fi lty y p ovi i an in l in r d 2 which 3 I3 should rise to the requisite point in as hort is inserted within the helices of the filament a. period of time as practical in order that too and extends from the bottom of the filament to long a wait may not occur between the turning a Point 3 adjacent the D thereofhe on.of the heating current and the operation of helices of the filament l closely Surround this the tube itself. In order thatthis condition be rod which thereby also maintains the spacing m evolved, I find that it is essential that no heatof the individual turns of the filament. The

insulating material should be placed between: the rod 2, together with the bushings 6 and II, are

filament I and the inside surface of the cylinder formed preferably of some material which'does 13.. In addition, it is desirable to so construct notreact appreciablywith the material of which thecathode that there are no bodies of high heat the filament, l is made at the temperatures capacity associated with-the heating filament. reach d by that. filamenli- For this p p I 45 n1 .order that anyvariations in voltage applied preferably use magnesia which is representative to the heating filament shall not affect the emitof a class of insulators of the type defined above. ting surface of the cathodait is further desir- Since: there is no insulating material between able that the filament! be entirely insulated the 'inner surface of the cylinder l3 and the .00 from. the cylinder 13. According to my. invenfilament l, and since the insulating rod 2 is of tion. I accomplisheach of the above results by comparatively low heat capacity, the structure supporting my heating filament l within the described above produces a particularly quick metal. cylinder in the manner to be described. heating cathode. In addition; the particular The cylinder l3 is provided at one end with manner of supporting the filament produces a an insulating bushing 6. This bushing is prostructure which is exceptionally strong and 5 which I have found in actual use is particularly free from microphonic noises.

I have found that if the turns of the filament are equally distributed throughout the length of the cathode, the temperatures at various portions of the emitting surface are different. In accordance with my invention I obtain a more uniform temperature distribution throughout the emitting surface by concentrating more turns in one section of the filament than in another. In the exemplification illustrated, more turns are concentrated in the upper portion of the filament than in the lower portion thereof. This particular distribution of turns is due to several reasons. First, the arrangement of the parts illustrated would tend to produce a higher temperature at the bottom of the coating II than at the top thereof if equally distributed turns were used. This is due partly to the fact that the coating I1 is not placed symmetrically on the cathode with respect to the filament. The heat generated in the leads 4 and and also the bottom turns of the filament I would heat the lower portion of the coating I! more than the top turns, and the loop 3 which would heat the upper end of said coating. By concentrating the turns as described, the temperature throughout the coating I1 is maintained uniform.

The gradual increase in pitch of the turns of the filament from the top to the bottom thereof also results in certain additional advantages. Since the slope of the turns at the bottom of the filament is very great, only a comparatively slight bend in the wires at the point where the leads 4 and 5 enter the holes I and 8 is necessary. I have also discovered that with such an arrangement, the turns at the lower end of the filament tend to hug the rod 2 very closely. Thus any tendency for the filament to bulge at this point is eliminated.

When an alternating current is used to heat a filament, magnetic fiuxes are ordinarily set up which would produce undesirable disturbing effects. In order to eliminate these fluxes, the filament I is non-inductively wound. This is done by making the filament in the form of a double helix, whereby the magneto-motive force of one turn is counteracted by an equal force of an adjacent turn. Thus the resultant magnetic field created by such a double helix filament is substantially zero.

In assembling the cathode, the insulating rod 2 is first threaded into the double helix filament I. The wires 4 and 5 are then threaded through the holes I and 8 through the bushing 6, and the fiat loop 3 is inserted in the slot I2 of the upper bushing II. This assembly is then inserted into the cylinder I3. I rigidly support said assembly within said cylinder, preferably by crimping the walls of the cylinder onto the walls of the insulating bushings. For this purpose the insulating bushing 6 is provided on opposite sides thereof with two longitudinal grooves 9, each of which is crossed by a transverse groove I 0. The insulating bushing II is likewise provided with-longitudinal grooves I5 crossed by transverse grooves I 6. The bottom of the cylinder I3 is crimped at the points I8 into the recess formed by the intersection of the grooves 9 and III, while the upper end of the cylinder is crimped into the recesses formed by the intersection of the grooves I5 and I6. By joining the insulating bushings, the filament and the metal cylinder in the manner described, all the parts are rigidly interconnected and the filament is definitely oriented within the cylinder. Thus the entire unit may be readily handled during subsequent operations with comparatively little danger of any relative displacement or distortion of the various parts.

The particular form in which the insulating bushings are made is such as to facilitate the manufacture of these bushings. The insulating material of which these bushings are made is such that it can only be worked while in a plastic state. While it is in this state, it is very fragile. It will be noted that all of the openings and grooves in both the bushings 6 and II, except for the small transverse grooves, extend longitudinally throughout the entire length of the bushing. Since these insulating members may be formed in their plastic state by extrusion, all of the longitudinal grooves and holes can be formed at this time. After the piece is extruded, a simple cutting or grinding operation is necessary to put in each of the grooves I6 and I0.

In order to provide an electrical connection to the emitting surface, the cylinder I3 is provided with a piece of nickel strip I4 welded to one end of said cylinder.

After the elements have been assembled as described above, the coating I1 is sprayed onto the cylinder I3. This coating may be any suitable one of the type described. I preferably apply the coating originally in the form of a solution of various carbonates, such as barium and strontium carbonate. These carbonates are subsequently converted into oxides, and the coating becomes a good emitter of electrons. cathode has been constructed in the manner described, it is tested for any possible defects, and is then ready to be assembled in the tube in which it is to be used.

Referring to Fig. 4 in which the cathode is shown in its assembled relationship with the other elements of a vacuum tube, I9 is the usual press of such a tube. This press consists of a plurality of wings extending in more than one plane. In the wings lying in one of said planes are sealed two supporting standards 29 and H. In the wings lying in anotherof said planes are two additional standards, not shown, supporting the anode 22. The upper ends of all of these standards project through an insulating plate 23 which is securely fastened to said standards by some such means as eyelets 24. A lower insulating plate 25, similar to plate 23, is fastened to the lower ends of said standards by eyelets 2B. The grid 21 is formed by a fine wire wound on the two grid standards 28 and 29. These standards are supported in insulating plates 23 and 25, the lower end of standard 29 being electrically connected to and additionally supported by a lead-in wire 30 sealed into one of the Wings of the press I9. The insulating plate 23 is provided with a central opening which in the assembled position closely surrounds the insulating bushing II. The insulating plate 25 is provided with a similar opening which surrounds the insulating bushing 5. The opening in the plate 25 is of such a size that although it receives the insulating bushing 8 and the strip I4, it is not large enough to pass the cylinder I 3. Thus the lower end of the cylinder I3 rests against the top of the insulating plate 25, thereby definitely positioning the cathode with respect to the rest of the electrodes. The strip I6 is welded to the standard 24 while the two ends 4 and 5 are welded to two heater leads 3I, but one of which is shown in Fig. 4. It will be noted that this method of support After the described above as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be all) given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical filament within said member and spaced from the walls thereof, the spaces between said filament and the walls of said member being substantially unobstructed, an insulating rod extending substantially the entire length of said filament within the helices thereof, said rod being prevented from moving transversely within said hollow member solely by the support afforded by said helices, and means for supporting the filament by its upper and lower ends.

2. A thermionic cathode comprising a hollow member adapted to emit electrons, a filament within said member and spaced from the walls thereof the spaces between said filament and the walls of said member being substantially unobstructed, said filament being in the form of a double helix, an insulating rod extending substantially the entire length of said filament within the helices thereof, said helices engaging said rod along substantially their entire length, said rod and helices thus affording mutual support, and means for supporting the filament by its upper and lower ends.

3. A thermionic cathode comprising a hollow metal tube, insulating bushings in the top and bottom of said tube, said tube being crimped onto each of said bushings, a helical filament within said tube and spaced from the walls thereof, an insulating rod extending substantially throughout said filament within the helices thereof, said bushings engaging and supporting the top and bottom of said filament, whereby said filament is maintained in place within said tube.

4. A thermionic cathode, including a hollow metal tubular member, a heating filament within said member, insulating bushings in the top and bottom of said tubular member, recesses in the surfaces of said bushings, said tubular member being crimped into said recesses said filament being supported at its upper and lower ends by said insulating bushings.

5. A thermionic cathode comprising an elongated hollow tubular member adapted to emit electrons, an elongated double helical heating filament within said member, lead-in wires connectedto said filament at one end thereof, the turns of said filament being more concentrated at the end opposite said lead-in wires, whereby said hollow tubular member may have a predetermined distribution of temperature along its length.

6. A thermionic cathodecomprising a hollow member adapted to emit electrons, a filament within said member and spaced from the walls thereof, said filament being in the form of a double helix, an insulating rod extending within the helices of said filament, said helices closely engaging said rod along its entire length, said helices and rod affording mutual support, said rod being prevented from transverse displacement within said hollow member solely by the support of said helices, and means for supporting said filament within said hollow member.

'7. A thermionic cathode comprising a hollow member adapted to emit electrons, a helical filament within said member and spaced from the walls thereof, the spaces between said filament and the walls of said member being substantially unobstructed, an insulating rod extending substantially the entire length of said filament within the helices thereof, said filament constituting the sole lateral support for said rod, said rod being mechanically disconnected from the rest of the cathode structure, and means for supporting the filament within said hollow member.

8. A thermionic cathode comprising a hollow metal tube, insulating bushings in the top and bottom of said tube, a helical filament within said tube and spaced from the walls thereof, an insulating rod extending substantially through said filament within the helices thereof, the top of said rod being spaced from the insulating bushing in the top of said tube, said rod being prevented from moving transversely within said hollow tube solely by the support afforded by said helices, said bushings engaging and supporting the top and bottom of said filament, whereby said filament is maintained in place within said tube.

PAUL T. WEEKS. 

